Shop floor scheduling is an important task in managing a production system. Various, sometimes conflicting objectives must be considered--for example, maximizing throughput; minimizing the average time orders spend in the factory, which reduces the amount of work in progress (WIP); maximizing the number of delivery dates met; maintaining inventory at a reasonable level; and maximizing the utilization of resources. One aspect of shop floor scheduling is controlling the release of pending jobs onto the shop floor--called lot release. Another aspect of shop floor scheduling--called lot dispatching--deals with the control of jobs already on the shop floor and is concerned with which lot of WIP is to be processed next when a machine becomes available.
Inherent in the establishment of a lot release strategy is a tradeoff between minimization of waiting times for WIP, and assuring that a sufficient backlog of work is available for the machines processing those jobs to avoid idle time at the machine between the completion of one job and the beginning of the next. Therefore a test is needed to answer the question at any given point as to whether a particular lot of work should be released or not. Although a number of such tests have been proposed, given the dynamic nature of factories, some deviation between predicted and actual results is inevitable. However, with a dynamic test guiding the lot release strategy, one can make changes in the strategy as strategy as the situation changes.
In a prior invention by the present inventors, patented as U.S. Pat. No. 5,170,355, entitled Apparatus And A Method For Controlling The Release of Jobs From A Pool Of Pending Jobs Into A Factory, issued Dec. 8, 1992, and assigned to Siemens Corporate Research, Inc. of Princeton, N.J., the assignee of the present invention, a method was disclosed for controlling the release of jobs from a pool of pending jobs into a factory comprising a plurality of machines processing jobs-in-progress in response to management objectives, based on a measure of "continuity index" as a test of whether a job is suitable for release or not. Such a continuity index (CI) is, informally, the ratio of job's actual processing time on one or more machines in the factory to the total time from release to completion of the job--i.e., processing time plus waiting time. In the referenced patent, it was shown that there exists an optimal CI value, CI*, and a range of values about CI* (the bounds thereof shown in FIG. 1 as CI*+ and CI*-), such that if a new job has a CI within this range, optimal performance can be maintained. This is illustrated in FIG. 1, which represents the product of machine utilization percentage (load factor) and percentage of orders completed on time plotted against CI.
From the curve shown in FIG. 1, it is apparent that the average continuity for all jobs should preferably be CI*. It also follows that each job which is released should contribute to this average in a way which is consistent with overall goals. Finally, as discussed in the referenced patent, the shape of the curve decides the sensitivity of the factory to changing CI. A flat top curve indicates that a wide range of WIP can yield optimal operating conditions and hence a good job mix can be achieved. A sharp peak, on the other hand, is a sign of sensitive environment, possibly due to the presence of certain critical and bottleneck work centers.
This invention relates to an improvement over the referenced patent wherein the lot release test is further refined to include (1) load balancing of the shop as a function of CI, (2) a relationship between job priority and CI and (3) the use of CI as a search guide for optimum release points. Applicants note that the prior art cited in the referenced patent is generally related to the improvement disclosed herein.